Re: Twitter bootstrap
I think just as with any library of scripts (and it is a library of scripts (CSS / js) and images) it’s to be considered a toolbox not an ultimate solution. There are already a few WordPress themes using it. If you wanted to use it to create a BuddyPress theme it could be a good option to speed up the process.
My only note of caution with things like that is to avoid the attack of the clones so to speak. Yes, it’s great but some variation is also great 
The web would be a truly boring place if we all had the same looking sites.
